What do you think is the biggest organ in your body? Is it your heart? Your brain? Your lungs?

Each of those organs is vitally important to you. But none of them is the biggest. Your skin is the largest organ you have. A grown-up's skin would be between 12 and 20 feet square if you could stretch it out flat. That's about the size of a tablecloth. It weighs a little more than one tenth of your body weight.

Your skin has several very important jobs. It holds you together, and it protects your insides. It helps keep your body at the correct temperature. And it feels things -- from the softness of a feather pillow to the warmth and graininess of a sandy beach.

Your skin is pretty tough. It has to be to withstand its daily meetings with wind, weather and germs. But sometimes germs do invade the skin. One of the things they cause is warts.

"Ugh!" you say. "Warts are awful." It's true that warts aren't very pretty to look at -- but luckily they are pretty harmless. If you have ever had any warts -- and most kids have -- you know that they tend to disappear as mysteriously as they appeared.

Many people think that you get warts from handling toads. Maybe that story got started because a toad's skin looks warty. But toads don't deserve their reputation. Warts are really caused by invisible germs called viruses. They get into cells on the outer surface of your skin, and multiply. Eventually, a small, hard, lumpy bump appears.

Warts most commonly grow on the hands and feet. Some may appear on the face, as well as on the elbows and knees. Sometimes the ones that grow on the bottoms of the feet -- called plantar's warts -- can be painful. When you walk, the hard wart presses up against the soft tissue of your foot. Ouch! When that happens, you may want to consult a doctor about whether the wart should be removed. It's not a very good idea to try to remove the wart yourself with preparations available in the drugstore. They can burn your skin if you use them incorrectly.

Another kind of bump or mark also appears on the skin. Called a mole, it is a little patch of cells that has an extra amount of the "dye" that makes your skin the color it is. This pigment, or coloring substance, is called melanin. Different kinds of people have different amounts of melanin in their skin. People with light skin have little melanin; people with dark skin have a lot.

When you were born, you probably didn't have any moles on your body. But within a few years they began to show up. Doctors aren't absolutely sure why people have moles -- but the location of the marks seems to be determined before a baby is even born. More moles appear as a person approaches adulthood. Then they may begin to fade. If you live a very long life, most of your moles may fade away altogether.

Many people think moles are attractive. They call them "beauty marks." Long ago, people at European courts used to glue false moles on their faces if they didn't have natural ones.

Like warts, moles are basically harmless. But they can cause trouble if you pick at them, or irritate them. If your moles become irritated, grow rapidly, or look different than usual, you should let your doctor know about it. It may be a good idea to remove a troublesome mole.

Otherwise, just let your moles be. They're another part of what makes you a unique person. Tips for Parents

Dr. Joseph Elliott Jr., a dermatologist and secretary of the American Dermatological Association in Charlotte, N.C., advises parents to keep an eye out for the appearance of warts on their children's skin.

"I think warts should be treated as soon as you know they're there," he says. "If they get large, or multiply, curing them can get traumatic for the patient. Plantar's warts, especially, require quick treatment."

As far as moles are concerned, Elliott recommends that you be aware of their shape and color, but not get overly concerned about them. The great majority will never require medical attention.

"An average person has 50 moles," he says. "We'd all be busy all day and all night removing them if we went after every one."

The moles you should be concerned about are those that change, which can be an early warning sign of certain cancers. Watch for:

Moles that change color -- particularly flat, tan moles that turn black.

Moles that develop rings around them, or get ragged, irregular borders.